1. Field of the Invention
The present invention relates to a semiconductor sensor device such as a semiconductor flow rate detector used for measuring the intake air flow of automobile engines and a method of fabricating the same.
2. Description of the Background Art
Description will be given on a semiconductor flow rate detector as an example of conventional semiconductor sensor devices.
FIG. 8A is a plan view of the conventional semiconductor flow rate detector, and FIG. 8B is a vertical sectional view taken along the line I--I of FIG. 8A.
With reference to FIGS. 8A and 8B, a semiconductor flow rate detector 1 includes a semiconductor chip 2 formed with a heater element 4 and a temperature-sensitive element 5 on a wafer such as silicon, a metal frame 3 for holding the semiconductor chip 2, leads 1 connected respectively to connecting terminals 2a of the semiconductor chip 2 through wires 6, and a base 8 made of resin for securely holding the frame 3 and leads 7.
The semiconductor flow rate detector 1 is installed in a fluid passageway such that a fluid, the flow rate of which is to be detected, flows in the direction of the arrow II of FIG. 8A.
FIG. 9 is a diagram of a circuit for measuring the flow rate of fluid by means of the semiconductor flow rate detector 1. Referring to FIG. 9, the operations of flow rate detection will be described hereinafter.
The heater element 4 has a positive coefficient whose internal resistance R.sub.H increases as the temperature rises. A resistive element 9 having a resistance R.sub.M is connected to the heater element 4 in series. The internal resistance of the resistive element 9 is unaffected by the temperature changes (i.e. has no temperature coefficient). A voltage V0 is applied across the elements 4 and 9.
A proportional value K is calculated from a potential V1 at a node (a) of the heater element 4 and resistive element 9 and the applied voltage V0 by the following expression: EQU K=V1/V0=R.sub.M /(R.sub.M +R.sub.H)
The heater element 4, which is disposed in the fluid passageway as above mentioned, transfers heat to the fluid, so that the temperature of the heater element 4 falls. As a result, the internal resistance R.sub.H of the heater element 4 decreases, and the proportional value K increases.
A comparator 10 compares the proportional value K calculated from the constantly detected voltage V1 with a predetermined constant K1 to send the comparison results to a voltage controller 11. The voltage controller 11 controls the voltage V0 applied to the heater element 4 as a function of the comparison results. For example, when the temperature of the heater element 4 falls resulting in K&gt;K1, the control described below is carried out. The voltage V0 is increased to increase a current flow in the heater element 4. The heat generated by the heater element 4 supplements the quantity of heat taken away by the fluid, whereby the internal resistance R.sub.H is returned to the initial value to achieve K=K1.
The quantity of heat transferred from the heater element 4 to the fluid is proportional to the flow rate of the fluid. A current detector 12 detects the quantity of current increased by the feedback control described above, and an operation unit 13 calculates the flow rate as a function of the current increment. The flow rate is displayed by a display unit 14.
When there is a difference between the fluid temperature and the temperature of the heater element 4 where the fluid flow stops and the flow rate is zero, the temperature of the heater element 4 falls and the situation is as if there were a practical fluid flow. In this case, the temperature-sensitive element 5 measures the fluid temperature to send a temperature signal to the operation unit 13. The correction is made such that the flow rate calculated by the operation unit 13 becomes zero.
In the conventional semiconductor flow rate detector 1 having such a structure, since the semiconductor chip 2 is securely held by the metal frame 3, the frame 3 conducts heat to other portions and the quantity of heat is taken away. This causes measurement errors and deteriorating detection accuracy. Another disadvantage is that an increasing heat capacity of the whole semiconductor flow rate detector 1 causes extremely slow responses when the flow rate of the fluid to be measured abruptly changes.
In addition, the wires 6 and leads 7 are exposed to the fluid. This causes the problems that the wires 6 are cut at a large flow rate and that bonding portions of the wires 6 are damaged. The reliability of measurement is therefore not sufficiently accurate.